This project seeks to quantify the mechanical and neuromuscular changes in the extensor mechanism of patellofemoral pain patients with patellar malalignment and with/without patellar taping. Patellofemoral pain is one of the most common complaints in sports medicine clinics. It is hypothesized that load sharing among the quadriceps, the contraction delay and mechanical actions at the knee of individual quadriceps heads, and patellar tracking are changed significantly by patellar malalignment. To determine in vivo isometric knee extension load sharing among the quadriceps components, electrical stimulation of different intensities will be used to activate an individual quadriceps head selectively at various contraction levels, and the relationship between the M-wave and isometric knee extension moment over different contraction levels will be established for each quadriceps head. Next, the subject will voluntarily generate certain levels of isometric knee extension torque and the same electrodes used to record the M-wave signals will be used to measure the EMG signals. The above relationship between the M-wave and extension torque of each quadriceps head will be used to relate an individuals muscle's EMG to the individual muscle's torque and thus determine load sharing among the quadriceps components. Changes in load sharing between vastus medialis oblique and vastus lateralis caused by patellar malalignment will be quantified. In addition, contraction delay (from the onset of activation of an individual quadriceps component to the onset of joint torque generated solely by the same individual quadriceps component) and mechanical actions of the individual quadriceps components will be determined by activating a quadriceps head selectively and measuring the resultant knee flexion-extension, abduction-adduction, and internal-external rotation moments. Patellar tracking induced during knee extension will be quantified noninvasively using an optoelectrical system and using a fluoroscope. Changes in the above properties induced by patellar malalignment and correction induced by patellar taping will be quantified using the above designs.